The Purity and Purification of Solid's Melting Points

The Purity and Purification of Solids Melting Points
CH-337
Abstract:
The purpose of this experiment was to find the eutectic point for the mixture of naphthalene and biphenyl. This was done by measuring the melting point of both pure substances and five mixtures of different ratios. The purpose of the second section was to identify an unknown sample by measuring its melting point and mixing it with two known substances and comparing them. The results suggest that the eutectic point is closest to the weight % ratio of 10/90 naphthalene to biphenyl and the unknown sample is acetamide.
Introduction:
One way to establish the purity of a substance is to measure its melting point. A pure substance will have a sharp melting range while an impure substance will have a more broad and depressed range. There is a point when a mixture of two substances will have a sharp melting range at a lower temperature than their respective pure melting points. This point is called the eutectic point. The purpose of this experiment is to find the eutectic point of a mixture of naphthalene and biphenyl and to identify an unknown sample by determining the melting point of a mixture with a known sample.
Experimental Section:
For this experiment, a melt-temp apparatus was used to determine the mp of pure and mixed substances.
To find the eutectic point of the naphthalene and biphenyl mixture, each person in the lab measured the mp of both pure biphenyl and pure naphthalene by warming them in the melt-temp apparatus at a rate of 2 °C/min and recording the temperature at which they begin melting and have completely melted. The class was then split in to equal groups to measure the mp of mixtures 10/90, 30/70, 50/50, 70/30, and 90/10. These were measured in the same way as the pure substances. The average mp was then recorded for each mixture and pure substance. The data was then plotted as a melting point diagram and used to determine the composition and temperature of the eutectic point.
To determine the identification of the unknown sample, the mp of the sample was measured followed by the melting points of the unknown individually mixed with two known samples with a similar mp.
Results and Discussion:
Table 1: melting points of different mixtures of naphthalene and biphenyl
Weight % Mole % Average MP (°C)
100% N 100% N 71.6-73.9
90% N/10% B 91.5% N/8.5%B 61.7-74.0
70% N/30% B 73.7% N/26.3% B 42.5-59.0
50% N/50% B 54.6% N/43.4% B 49.7-61.0
30% N/70% B 34.0% N/66.0% B 62.5-66.5
10% N/90% B 11.8% N/88.2% B 71.5-74.0
100% B 100% B 82.6-85.2
The literature value1 for the mp of naphthalene is 80.26 and biphenyl is 69.2. The results suggest that the eutectic point is somewhere around 11.8% naphthalene 88.2% biphenyl since the smallest melting range at this point. There was a fairly high standard deviation for the melting points, which may have been caused by the fact that there were several different devices used to measure the temperature. Also, it was a little difficult to accurately pinpoint the exact temperature the melting started and ended and different people probably have different perceptions of this.
Table 2: Melting Points of Unknown Sample and Mixtures
Sample MP (°C)
Unknown 91-94
Unknown + Acetamide 91-94
Unknown + Acetanilide 79-90
This data suggests that the unknown sample is acetamide since the mp did not change when mixed. This portion of the lab was pretty straightforward and there was not much chance of an error. The measured mp might differ slightly from the literature values but since we used the same equipment throughout this section the results were still accurate.
Conclusion:
The data suggests that the eutectic point for the mixture of naphthalene and biphenyl is around the mole % ratio of 11.8/88.2 naphthalene to biphenyl. The identity of the unknown sample was identified as acetamide.
Questions:
A finely pulverized sample has more surface area, which makes it easier to melt. Less surface area will result in the sample taking a longer time to melt.
A broad melting range for a pure substance could be a result of not pulverizing the sample to create enough surface area. A sharp melting range for an impure substance could be because the impure substance is a mixture at its eutectic point
The mole % ratio of 2 substances where the melting point is at its lowest.
The crushed glass, filter paper and plaster will increase the melting range due to being impurities and they will change the appearance of the melt because they are not soluble in the acid.
I would make 2 mixtures of the unknown sample mixed with hippuric acid and the other with succinic acid and measure the mp of each of these mixtures. One will be the same mp as the pure sample and the other will have a lower mp. The one with the same mp will be the winner.
References:
Karl Griesbaum, Arno Behr, Dieter Biedenkapp, Heinz-Werner Voges, Dorothea Garbe, Christian Paetz, Gerd Collin, Dieter Mayer, Hartmut Höke “Hydrocarbons” in Ullmann's Encyclopedia of Industrial Chemistry 2002 Wiley-VCH, Weinheim.